Thermopower and resistivity of La-doped thallium 1201 and bismuth 2204 superconductors

Abstract

We have measured the thermopower and resistivity of two series of cuprate superconductor, Tl_0.5Pb_0.5Sr_2-xLa_xCuO₅ and Bi₂Sr_2-xLa_xCuO_6+y, which possess a single CuO₂ plane. In each series, the hole concentration can be varied by changing the La content, permitting investigation of the overdoped, maximum-T_c and underdoped regimes. The thermopower follows a pattern similar to that seen in superconductor series with more than one CuO₂ plane, namely an approximately linear decrease as temperature increases well above T_c. However, the thermopower shows a minimum above T_c followed by a maximum as temperature increases for samples with a small thermopower and high T_c. The magnitude of thermopower as a function of T_c is in general agreement with the universal behaviour found for other cuprate superconductors.